A major effort in contemporary biological research is to identify loci that control developmental decisions. In addition, disturbance of the normal differentiation process can contribute to tumorigenesis. Determining the key regulatory mechanisms involved in generating and maintaining the differentiated state is important for understanding both normal development and disease. This is a proposal to investigate the control of cell type determination in mammalian cells. By utilizing myoblast cell lines grown in vitro, and a combined somatic cell and molecular genetic approach, the regulation of the MyoD family of myogenic regulatory genes will be analyzed. Intertypic hybrids will be assayed for activation and repression of myogenesis. Genetic studies of this nature have been utilized to identify positive and negative regulatory interactions in a number of experimental systems. This project has four goals: 1) define the role of negative regulation of MyoD in 10T1/2 X fibroblast hybrids, 2) determine whether additional fibroblast chromosomes activate muscle, fat, or cartilage phenotypes in 1OT1/2 cells, 3) determine whether MyoD is repressed in cis or in trans in 1OT1/2 cells, and 4) examine the genetic basis of extinction in 1OT1/2 X L6 myoblast hybrids. By identifying and ultimately isolating these unique regulators, we can begin to define the genetic mechanisms involved in the determination and differentiation of muscle cells. An understanding of the regulatory circuits which control the myogenic lineage will give important insights into the perturbations of gene expression that result in abnormal muscle development and tumorigenesis.